**Core Concept:** The normal menstrual cycle is a hormonal process that regulates reproduction in women. The cycle consists of two main phases: the follicular phase and the luteal phase. Progesterone is a hormone produced during the luteal phase, which helps prepare the uterine lining for potential implantation of a fertilized egg.

**Why the Correct Answer is Right:** Progesterone is synthesized and secreted primarily by the corpus luteum, which is formed from the ruptured follicle following ovulation during the follicular phase. The corpus luteum produces progesterone in response to the surge of luteinizing hormone (LH), which occurs near the peak of the surge of follicle-stimulating hormone (FSH). Progesterone's main role is to increase the thickness and vascularity of the endometrium (lining of the uterus) to prepare it for implantation of a fertilized egg.

**Why Each Wrong Option is Incorrect:**

A. Ovary: Although progesterone is produced by the corpus luteum in the ovary, the source of progesterone during the normal menstrual cycle is the corpus luteum, not the ovary itself.

B. Ovarian stroma: While progesterone is synthesized in the ovarian stroma, the correct answer should focus on the corpus luteum, which is the specific source during the menstrual cycle.

C. Placenta: Progesterone is present in pregnancy due to the placenta, but during the normal menstrual cycle, it is produced by the corpus luteum.

D. Plasma: While progesterone is present in plasma, the correct answer should focus on the corpus luteum as the source during the menstrual cycle.

**Correct Answer:** C. Placenta

**Clinical Pearl:** Progesterone plays a crucial role in maintaining an appropriate uterine environment for potential embryo implantation, and its levels increase during the follicular phase (second half) of the menstrual cycle to prepare the uterus for pregnancy. If pregnancy does not occur, progesterone levels decrease, leading to uterine contractions and menstruation.